The present invention relates to an electric double layer capacitor using polarizable electrodes made of activated carbon fibers and a method for producing the same.
As shown in FIG. 1 of the accompanying drawings, the electric double layer capacitor basically includes a pair of polarizable electrode units formed by activated carbon layers 1 and 2 and collector electrodes 3 and 4, and a separator 5 interposed between the polarizable electrodes. The activated carbon layers 1 and 2 and the separator 5 are impregnated with an electrolyte.
The electric double layer capacitors of this basic construction are roughly divided into the following three types of specific constructions. As shown in FIG. 2, the first type is constructed by applying a mixture 6 of activated charcoal powder and an organic binder such as fluorine resin powder onto aluminum nets 7, coiling the aluminum nets 7 together with separators 8, placing these components in a metal can 9 and hermetically sealing the can 9 with a rubber sealing cap 10. The coiled assembly is impregnated with an electrolyte composed of an aqueous solution of sulfuric acid or the like, or an organic mixture of tetraethyl ammonium perchlorate and propylene carbonate or the like as mentioned previously, and numerals 11 and 12 designate electrode leads. The second type, shown in FIG. 3, uses viscous paste 13 and 14 prepared by mixing an aqueous electrolyte and activated charcoal powder and it includes an interposed separator 15, insulating rings 16 and 17 and conductive elastic electrodes 18 and 19. With the first and second types using activated charcoal powder as the polarizable electrodes, the former features high breakdown voltage and the latter features reduced internal resistance. In addition to these types, a third type (Japanese Laid-Open Patent Application No. 55-99714) which we have invented employs activated carbon fibers as polarizable electrodes.
In FIG. 4 showing the third type capacitor, activated carbon fiber fabrics 20 and 21 which are respectively formed with metal sprayed coatings 22 and 23 are arranged opposite each other through the intermediary of a separator 24 and are hermetically assembled with a gasket 25 and metal cases 26 and 27. Since this type can be formed into a flat coin shape, is easy to manufacture and is small in size and high in capacitance, the third type is suitable for miniature circuits for back-up use in microcomputers.
Then, with the capacitors of the type using an activated carbon fabric, if the fabric is produced by carbonization and activation processes, the manner of weaving the fibers into the fabric has an important effect on the characteristics of the capacitor having the thus formed fabric. More specifically, the higher the proportion of the fibers entering the inside of the fabric is, the harder will it be for the activation gas (e.g., water vapor, CO.sub.2 or HC) to sufficiently penetrate into the surface of the fibers at the inside during the activation thus impeding the progress of the uniform activation for the fabric on the whole. Therefore, in order to satisfactorily promote the uniform activation of the fabric on the whole, it is impossible to obtain the desired specific surface area per unit weight unless the activation temperature is increased extremely or the activation time is increased. However, if the activation temperature is increased or the activation time is increased, the strength of the resulting activated carbon fabric is descreased considerably thereby rendering the activated fabric unsuited for use.
It is an object of the present invention to provide an electric double layer capacitor having polarizable electrodes constituted by activated carbon fibers which are activated fully and have a large specific surface area.